Typical coal-fired electric power generation plants produce hundreds of tons of coal ash each day. The disposal of ash can be extremely complicated not only due to the nature of the ash but also due to the extensive transportation and disposal systems which are utilized to meet numerous environmental regulations. Ash produced in the burning of coal is dry (fly) ash, an extremely fine powder-like substance, and bottom ash. Dry (fly) ash cannot be simply dumped but must be carefully conditioned with water to prevent material dusting and improve ash disposal site compaction. The disposal of the ash is further complicated by the fact that the conditioned ash often creates an exothermic and/or pozzolanic reaction that can produce a concrete-like end product. Consequently, time is an important factor in dumping conditioned ash.
In order to dispose of the ash, the dry ash which is the bulk of the ash produced is transported from the power plant boiler to a dry ash storage silo. When an empty ash haulage vehicle is available, the ash is allowed to flow from the dry ash silo to an ash conditioner such as a pug mill which mixes the dry ash with a limited amount of water. The amount of water added is typically manually controlled. It will be appreciated that ash conditioning pug mills are substantial pieces of equipment which will typically produce on the order of 250 tons of conditioned ash per hour. As conditioned ash is produced, the conditioned ash is loaded directly into an ash haulage vehicle for transportation to an ash disposal site. After each load of conditioned ash is produced, the ash conditioning pug mill is shut down, waiting for the next empty ash haulage vehicle to return for another load of conditioned ash.
The ash conditioning process is often a source of loading delays. Studies have indicated that the time to condition ash in the conditioning process for a typical conditioned ash haulage vehicle constitutes 40-60% of the vehicle's haul cycle time. In addition, manually starting and stopping an ash conditioning pug mill not only creates extensive delays, but also affects the quality and consistency of the conditioned ash. It will be appreciated that the proper moisture content and quality of the conditioned ash is obtained by manually adjusting the rate of water addition to the dry ash depending upon the quality and characteristics of the incoming dry ash. The intermittent use of the conditioning pug mills resulting from the constant starting and stopping of the ash conditioning pug mills and other delays associated with each load of conditioned ash produced makes it extremely difficult, if not impossible, to control the quality of the conditioned ash for a single load. In some instances, the proper characteristics may not even be obtained until the ash haulage vehicle is filled and the ash conditioning pug mill is about to be shut down, pending the arrival of another empty ash haulage vehicle.
Other significant and costly loading delays can result from the layout of the generation station relative to the ash disposal site. With nearby, non-contiguous ash disposal sites becoming increasingly necessary, increased delays may occur when the ash haulage vehicles are unable to quickly navigate between the ash conditioning pug mill and a nearby, non-contiguous disposal site. These delays could occur when, for example, the ash haulage vehicle is unable to navigate over rough or uneven terrain. Other delays occur when ash haulage vehicles must cross obstacles such as streams or public roads. The presence of a stream may require a costly bridge to be built to allow large ash haulage vehicles to cross the stream, and the presence of a public road to cross may require additional personnel to direct traffic, increased liability insurance, and smaller ash haulage vehicles which can navigate the public road.